I spent a long time repairing the flow module section of this RT-200 gasflow analyzer (RT-203M). Today is the time to summarize the situation. I have completed the initial calibration and conducted verification tests for low flow rates; high flow rates have not yet been tested, but I anticipate there shouldn't be any major issues.
Let me describe the sequence of events from the beginning. The main issue with my RT-200 unit is zero-flow drift in the RT-203M gasflow module. The specific symptom is as follows: after connecting the power, entering the function code, pressing Enter to access the test mode, and pressing the zeroing button, the display shows "0.00 LPM"; however, the flow rate reading changes significantly over time, even though no airflow is entering the high-flow or low-flow test ports. Enter the "30" function test code and press the Enter key; you will see the displayed number changing slowly. In other words, it is not possible to achieve the stable display of the voltage value corresponding to "385" (±0.005 VDC) as described in the manual. I ultimately discovered that the zero-point drift was caused by the aging of the 125PC05D1 pressure sensor.
The 125PC05D1 is an older model of differential pressure sensor; it has been discontinued, and an identical replacement model is no longer available on the market. I reverse-engineered the original signal modulation board and found that R1123 (VBSET) is used to adjust the pressure sensor's sensitivity; R1116 (ZERO) is used to zero the sensor's S+ or S- terminals relative to GND; and R1111 (XDBAL) is used to zero the offset between the sensor's output pins, S+ and S-.R1102 (NULL) is used to adjust the final analog output voltage.
At a bias voltage of 10.0 V, the sensitivity of the 125PC05D1 pressure sensor is 70 mV/5 psi (14 mV/psi) or 2.0 mV/kPa. The gain of the original signal modulation board is vo/vi = (1 + (2 × 24,800 / 100)) = 497. Therefore, the sensitivity of the original module is 2mv/kpa × 497 = 994 mV/kPa.
I selected the XGZP192-010G as the new replacement pressure sensor.The sensitivity of the XGZP192-010G pressure sensor is 4 mV/kPa.If the XGZP192-010G is used, the instrumentation amplifier circuit in the original design requires a gain of 994/4 = 249. At the same time, resistor R1101 needs to be replaced, changing its value from 100 ohms to 200 ohms. Since the REF01 is a 10.0V voltage reference chip, the optimal condition is for the actual bias voltage to be slightly below 10.0V; therefore, I set R1101 to 195 ohms.At the same time, resistors R1118 and R1113 need to be changed to 24k and 27k.Only in this way can the new XGZP192-010G pressure sensor operate at its optimal performance level.
